It's easier to back up a motor-home than it is with a car and van so I'd hope that a motor-home driver would use a bit of intelligence and not try and force their way through.
However that said, I've seen some pretty aggressive motor-homers pushing to get a van to back-up

One of the reasons for not allowing people on site early is to enable others to leave site - in some cases the access roads may not easily accommodate vans passing each other - Rowntree Park being a case in point and I believe Chatsworth.
I'm sure a chat with the site warden would clarify the situation for the specif site.
The collective wisdom of this forum, although well meaning is not the same as getting the information and agreement of the site itself.

There was a previous thread where some had problems with Kampa Limpets causing distortion on the sides of vans.
It the end it's entirely up to you but TBH I'd rather not risk it. I'd be pretty well P'd off with a carbuncle or two on the van side because once there you can't get rid of them

Yes, the only reason for flexible ducting is that it's usually easier to install and lighter in weight. Be a bit wary close to the heating unit though as the very hot air coming form the heater could melt or distort the plastic pipe. Some heaters can get extremely hot.
As Stevan suggests it's a good idea to insulate any that is installed outside of the living area like underneath the van/vehicle.

No damage, just annoyance. How do you persuade people who think they are immune. Perhaps you ban them from parking in the car park instead.
A lot of the problems with society nowadays is inconsideration, arrogance and the fact that there don't seem to be any sanction against bad behaviour.

My bro used to work in a place where inconsiderate parking was rewarded with an A3 sticky label stuck on the windscreen with very tenacious glue which could take half an hour or so to get it all off.
It worked well as it deterred both the perpetrator and those who saw it it.

In most cases I've never found Captain Tolley's to be any good unless it's to try and seal minor leaks between a window and frame.
I can't see it doing much to improve a crack in a perspex/acrylic window especially if it it's a bit flexible. CTs tends to go hard so will probably do a temporary repair but then crack when it hardens.
I tried it on my boat and so did my mate and it only worked between the window and frame or frame and the substrate it was fixed to. In the end we had to do a proper job to solve the problems.

It's therefore up to the site owner to admonish him and make him shift it, or failing that shift it himself and charge the van owner for the time and effort involved. Or better still charge him for two plots

Have a shufti here https://www.centurybatteries.com.au/content/documents/battery-Talk/issue-2-battery-Talk-battery-charging.pdf & here http://support.rollsbattery.com/support/solutions/articles/4345-agm-charging
The problem with AGM batteries is that they require a slightly higher charge and absorption voltage when charging so they frequently get under charged especially when just bunged onto a caravan charger which doesn't ever get anywhere near the charge and absorption voltages.
So they seldom get charged to even 80%. This will quite rapidly damage them due to plate sulphation. Even normal flooded cell or maintenance free lead acid batteries should be given a decent charge now and again otherwise they lose capacity due to sulphation.
Floating a battery at 13.6-13.8v is OK but they do need to be fully charged first which means getting them up to between 14.4 and even nearly 15v depending on the battery type and construction.
A similar problem has been happening with cars fitted with stop/start technology where battery charging regimes are designed to charge certain battery type like silver calcium or EFB batteries. Fitting unsuitable batteries which don't get properly charged (either under or over charged) can rapidly finish them off.
Most reputable battery manufacturers or suppliers will proved advice and data sheets for their batteries and it's interesting to see that even they differ in their recommendations for similarly described (AGM, VRLA, Gel, Flloded or even EFB) so it always best to try and find out if you can rather than just using generic info.

No, when I retired almost exactly 18 years ago in March 2001 I got £500 worth of Sony TV as a leaving prezzie , so I could put my feet up and watch TV in comfort without worrying too much about time. It was 32" and weighed around 55kg.